But corporate tax consultant Robert Willens, head of Robert Willens LLC, sees no reason Citigroup couldn't have done a tax free spin off. Under such a deal, Citigroup would simply have split up into two separately listed companies, avoiding any corporate tax.

Willens cannot conceive of how such an option would not have occurred to Bain, unless Citigroup made it clear from the outset it wanted the firm to come to a different conclusion.

"It's not like the firm they hired to look at the tax aspects said 'Oh God--geez, we forgot: You can do a spin-off!' No--there's no way that would have happened," Willens says.

In an email, Bain spokeswoman Cheryl Krauss referred "any questions regarding our work," to Citigroup. That wording sounded like a confirmation that Bain did indeed perform the analysis, as Reuters reported, but Krauss did not respond to a follow-up email seeking clarification.